Heh heh - I think anyone in a position to answer this question (anyone you should listen to, anyway) probably knows what matte painting is...

Matte paintings most certainly are used in games - I've been offered work doing them in the past. Not all games, obviously - but as games and films move closer together, it's a technique that's being used. I think racing games are probably the most obvious application, but there are plenty of others, as well as in-house cutscenes.

Limiting yourself to 'matte painting for games' only is probably going to be a problem (especially as a junior), but it's not an either/or question.

A more important point is that outside of game design a 'gamer' and a 'game developer' are not in any way related. Just because you enjoy playing them doesn't mean you will enjoy making them, and I know a LOT of people who have made that mistake.

I would agree with Alex, there is a huge difference between enjoying playing games and watching film / tv to working on them. All of those industries are very high pressure wotking environments, and whilst they are often very rewarding, people can get very disheartened when they realise that making games isnt always as easy and fun as playing them.
That said, only you can really answer that question. If you want to do it as a career then make sure you work hard at it, its a competitive industry.
And yes, game cinematics do use a lot of matte painting, see www.thunig.com

What are the usual prices you charge for matte paintings,ranging from simple mattes/backgrounds(digi painting and photomanipulation without 3D renders or camera movement)to complex matte paintings that include 3D renders and camera projection techniques.

Salary / pricing is a bit of a touchy subject - it's a bit like a company phoning up their direct competitor, and asking what their profit margin is.

It's also based on so many different things - from your own personal skills, to how much you like/hate working for a client, to the living costs in your particular part of the world. Only you know what your 'basic subsistence' level is, and how fast you can complete a particular project.

However, since I know that's a really annoying answer:

A junior in the UK with little experience could be earning 15-20K (sterling) per year full time. If you are freelancing, you won't be working all the time, so the rate will go up to compensate for this.

At the other end of the scale, VERY experienced artists on short freelance contracts can occasionally be paid as much as 400UKP / day.